Two types of standards for communication interfaces for IC cards are currently used, or are going to be used in the future. One of the standards is for a contact-type card (clock: 3.5 MHz, 4.9 MHz) for use in the finance industry, such as ATM cards. The other one is for a non-contact-type card (clock: 13.56 MHz) for use in the transportation industry, such as cards for used for trains, buses and the like.
On the other hand, a combination-type IC card in which the functions of the cards of those two systems (standards) are integrated has been developed and in a market. High usability is attained by such combination-type IC card, which includes two types of interfaces, namely, a contact-type interface and a non-contact type interface, because only one combination-type card is necessary to deal with both the system of the finance industry in which the existing contact-type interface is used, and in the system of the transportation industry in which the non-contact-type interface is used.
Incidentally, the combination-type IC card requires means for judging which one of the contact-type interface and the non-contact-type interface is to be used in an environment in which the combination-type IC card is to be used. Conventionally, the combination-type IC card reads a clock from a machine to which the combination-type IC card is about to communicate, in order to judge whether the machine is the contact type or the non-contact type. In short, the judging is carried out by the combination-type IC card by reading the clock from the machine.
Specifically, for example, the Japanese Laid-Open Patent Application, Tokuhyo, No. 2001-502456 (published on Feb. 20, 2001) discloses an art in which presence and absence of clock in a signal from an antenna coil is used as a function so as to judge whether communication is the contact-type or the non-contact type. When the clock is present, it is judged that the communication is the non-contact type. When the clock is absent, it is judged that the communication is the contact type.
In this case, it is necessary to wait a certain length of time until operation is stabilized, for the non-contact type card, which starts operating with a signal and an energy supplied thereto from the machine in a form of an electromagnetic wave. Thus, the card of the non-contact type is so arranged as to be capable of normally operating after the judging in the type of the communication.
However, the antenna coil of the conventional combination-type IC card is a parallel resonance circuit, which is oscillated not only when receiving the clock of not 13.56 MHz for non-contact type communication, but also when receiving a clock of another frequency. Thus, it is a problem that the judging as to whether the communication is the contact type or the non-contact type cannot be performed stably by judging whether the clock for the non-contact type communication is present or absent. Specifically, in this art, even if a clock of another frequency but not the clock of not 13.56 MHz for non-contact type communication is received, there is a possibility that it is judged that the clock for non-contact type communication is present. This leads to erroneous operation of the combination-type IC card.